The invention relates in general to temperature measurement devices, and in particular, to a temperature measurement device using a dual thermocouple arrangement with an unshielded thermocouple and a shielded thermocouple to estimate and compensate for incident radiation when determining a temperature of a gas stream.
Aspirating pyrometers, also known as suction pyrometers, are used to measure extraordinarily high temperature gas streams, such as up to 2800 F. For example, high temperature pyrometers are used to measure temperatures in combustors of gas turbines and in gasification applications. Combustor temperatures must be measured or estimated to properly evaluate performance of the combustor and gas turbine. However, no commercially available pyrometer has been able to withstand the hot gas temperatures in advanced, high-firing-temperature, combustion-test stands. Pyrometers also have limitations in measuring the local temperature of gases.
Because prior pyrometers cannot withstand such hot gas temperatures, gas analysis has been used to estimate the temperature in combustors. However, direct temperature measurement, such as with a thermocouple, is preferable to gas analysis because direct temperature measurement can provide a near actual gas stream temperature instead of an estimated temperature. In addition, gas analysis is complex, expensive and slow relative to thermocouple measurement. Accordingly, a long-felt need has existed for devices that directly measure temperatures of gas streams in combustion test stands.
A thermocouple measures the temperature of the thermocouple junction. Thus, to measure the temperature of a gas stream, the thermocouple junction must attain the same temperature as the gas stream. On a superficial level, it may seem that positioning the thermocouple in the gas stream should cause the junction to attain the temperature of the gas. However, the thermocouple temperature is not a function solely of the gas temperature surrounding the thermocouple. The temperature of a thermocouple in a gas stream is a function of the steady state condition where the rate of heat transfer to the thermocouple junction balances the rate of heat transfer from the junction. The temperature of a gas stream around the junction is only one factor affecting this steady state temperature condition.
The steady state temperature condition of a thermocouple junction in a gas stream results principally by the balancing of four known thermocouple phenomena: (1) heat transfer from the gas stream to the thermocouple by convection, (2) heat transfer between the thermocouple and its surrounding by radiation, (3) heat transfer between the thermocouple junction and its wires by conduction, and (4) conversion of kinetic energy to thermal energy at the boundary layer surrounding the thermocouple.
The accuracy of measuring gas stream temperatures with a thermocouple depends to a large extent on how close the thermocouple junction can be brought to the gas stream temperature. The devices are designed to match the temperature of the thermocouple junction to the gas temperature. For example, an aspirating thermocouple increases the effect of convection heat transfer from the gas stream to the thermocouple junction by increasing the gas stream velocity across the thermocouple. Aspirating devices enhance the convection heat transfer between the gas stream and thermocouple to cause the thermocouple junction to better attain the actual gas stream temperature. Aspirating thermocouples enhance the convection heat transfer to diminish the effect of the other phenomena affecting the thermocouple junction temperature.
Aspirating thermocouples are designed to reduce the phenomena, other than that of convection between the gas and thermocouple that affect the steady state temperature condition of a thermocouple junction. The effect of incident radiation on a thermocouple junction from its surrounding environment influences the steady state temperature condition of a thermocouple. The effect of incident radiation on a thermocouple varies in proportion to the fourth power of temperature. Accordingly, the effect of incident radiation predominates the other phenomena affecting the steady state condition of a thermocouple. In one example, the incident radiation produced by a gas stream at extraordinarily hot temperatures, such as in the combustors of gas turbines, a gas stream in gasification applications, and the like, predominates the other phenomena affecting the steady state condition of the thermocouple. Because the thermocouple must be exposed to the gas stream to measure the temperature of the gas stream, it is impossible to prevent some incident radiation produced by the gas stream from effecting the temperature measurement of the gas stream. However, it is highly desirable to reduce the effect of incident radiation that is produced by the gas stream distal to the thermocouple on the temperature measurement of the gas stream.